Converting the 30″ Rubber Powered Dumas Piper J-4 to Electric RC – Part 2

Converting the 30 Rubber Powered Dumas Piper J-4 to Electric RC – Part 2

Last time I finished up with just enough work done on the fuselage to permit me to show you a trial mounting of one wing panel on the modified cabin assembly and the new 1/16″ plywood wing carry-through spar I designed for this conversion project. This time around I’m going to add the definitive “Piper shape” to the fuselage by assembling formers, stringers, and the cowl, as well as showing you how I converted the original rubber-powered free flight fuselage to accept a brushless motor and ESC along with a receiver and servos.

A laser cut part B-2 forms the outline of the rear window on each side of the fuselage. (On full scale light airplanes this is called a “D” window) Because I have redesigned the upper fuselage side to include a built-in wing root baseplate, the existing D-window frames must be trimmed back to fit the revised structure. You can see where I have added a pencil line to mark where to make the cut.

 

With that done, the assembled D-window frame looks like this. Notice that the frame, along with the triangular B-6 window frame parts ahead of it, fits flush with the top edge of the upper 1/16″ x 1/8″ fuselage stringer.

 

The landing gear comes next. Remember that I’m building the fuselage as nearly as possible in the same sequence as called out by the kit instructions. I’m going to use the furnished .041″ steel wire for the primary landing gear strut/axle. Here you can see that part bent to shape per the plans, along with the three 1/16″sheet landing gear mounting plate parts B-7 and B-8 that I will also use as designed. Note: As I will use bigger (thicker) wheels than the ones provided, I have left the axle portions of the gear wire extra long. I’ll trim them to an exact fit later.

 

Here’s the beginning of the landing gear mount assembly. I’m also doing this part of the job per the plan.

 

Again per the plan, I’m assembling the laminated gear wire-mounting plate, using clothespin clamps to maintain alignment. I used fast ZAP as an adhesive on this joint.

 

Because my converted J-4 is going to weigh several times what it would have as a rubber job, I added wire landing gear components that pretty much match the full scale layout. The rear/auxiliary strut was bent from .036″ wire to match the main strut shape and fits between laminated 1/16″ balsa plates that fit against the  3/32″ sq. balsa cross member to the rear of the main strut. (It happens that this very closely matches the full scale aux. strut position.) I also made an .036″ wire cross strut to match the full scale gear. Here you can see how I have assembled both the main and auxiliary strut mounts in place, used a copper wire wrap to hold everything together, and then built a simple jig to hold the entire assembly in alignment for soldering.

 

Sorry…I finished the soldering job off-camera so I could give my full attention to getting it right. (I used 95/5% tin-silver solder with acid flux). Here’s what the finished assembly looks like from above. You can see my scale reference drawing behind the model. (I’ll show you better images of the solder joint in a bit.)

 

The rubber powered model is designed to have the three cowl front/face parts laminated together and then left separate from the main fuselage so it can be removed for rubber motor access. I’m going to deal with electric motor access differently, so I am going to assemble this laminated cowl front permanently to F-1.

 

Now I have to catch up with a step I left out when I modified the wing to become a two-piece structure. The leading edge center block/upper windshield fairing was supposed to be part of the wing, but the center section is now part of the fuselage (as it is on the full scale airplane) so now it goes here. I trimmed the width of the existing balsa block to fit between the new wing root base plates, made the rounded cutout in the center to match the plan (and the scale reference drawing) and ZAP’d it permanently in place. Now I’m using a No. 11 blade to rough-trim the squared-off block to match the finished leading edge airfoil shape. I’ll come back later to finish the job with my sanding block.

 

Bottoms up please! I’m adding the 1/16″ sq. stringers to the underside of the nose/cowl exactly per the plans EXCEPT that I am substituting 1/16″ sq. basswood for the balsa strips provided in the kit, which were a bit on the soft/light side…OK for a freeflight job but likely to crack or deform under the tension of shrinking covering or the stress of normal handling of what has become a heavier model. Changes like this will always be a judgment call on rubber power kit conversions. I prefer to risk a bit of extra weight in favor of the durability I want to build into a model I plan on flying a lot and keeping for a while.

 

Here’s a better look at the cross strut I added to the landing gear assembly. Now you can see better what my finished solder joint looks like. The  cross strut attaches to the fuselage bottom at the centerline on the full scale airplane with a metal fitting. Replicating that on this little model would be needlessly complex AND heavy, so I’m depending the V-bend formed into that wire strut to flex under loads and adding a reinforcing plate made by inlaying a bit of 1/8″ basswood shaped to fit. The final covering job will lie over the stringer with the plate beneath it to handle rebounds from the strut.

 

More stringers…and more of that 1/16″ sq. basswood. If you look carefully you can see that I’m putting light pressure on the aft end of the near stringer with my thumb to induce the gentle bend necessary to fit the curve that is part of the fuselage side profile and defined by the relative heights of the formers.

 

The assembled cowl top stringers look like this. I left the aft ends a bit long so I could use a razor saw to cut them off after assembly for a better fit.

 

I’m adding the front cowl side plates (B-2) per the plans. You also get another close look at the soldered landing gear wire assembly here.

 

This is the top/rear of the fuselage with the first of the 1/16″ sq. basswood stringers in place. There’s a lot of work left to do on all those 1/16″ balsa sheet formers before I’m done with them.

 

This is one of the most critical steps you can take if you want to build a good-looking model of a fabric covered airplane. (This applies to ANY covering…tissue, silk, polyester, or film…that bridges open structure between stringers. RELIEVE …”scallop out”…every former, bulkhead or other cross piece except at the ends of any open bay of covering. The front and rear structure and each pair of stringers are supposed to define the surface contour/curvature of the fabric. The formers are not supposed to show, and if you let them, they’ll appear as crude bumps.
You’ll ask me, what if I’m building a model of a plywood or metal skinned airplane like a P-51? Many kit designers assume that the former edges against the covering will “add shape” to simulate a convex metal surface. Guillow’s fighter models provide an excellent example of this. My opinion is that the result of doing it that way would create a model I wouldn’t want anybody to see.

 

This is a good look at the top fuselage formers contoured to permit the covering to assume that correct curvature between stringers. (By the way, if you’re familiar with kit, you’ll notice that I’ve added a top/center stringer …in accordance with the scale references…to permit doing the scale covering job you’ll get to see later on.) Now I’m using a 100-grit sanding block – gently – to blend the outer faces of all the side structure (D-window frames, stringers and longerons) into a covering base that will permit the tissue to lie FLAT against it with no bumps, warts, or other spoilers.

 

Here’s what might look like an exception to the rule I stated with Image 16. I’ve added some 1/16″ balsa sheet scrap between those belly stringers at the place where the rear/fixed 1/32″ balsa sheet nose cowl (the “boot cowl”) will end. Later I’ll sand these to match the contour of a sheet of balsa bent to fit around the perimeter described by the stringers. Without that extra structural reinforcement the finished sheet balsa boot cowl surface would flex and crack under normal handling stresses.

 

Time for some sheet balsa! The entire nose and engine cowl on the J-4 is curved/bent/formed sheet aluminum. For a model of this size, in the absence of a molded plastic or fiberglass cowl, sheet balsa is the best way to go. Having learned some things from doing this before, I decided that the entire top cowl surface from F-1 to the instrument panel between the two upper fuselage longerons can be covered with one piece of 1/32″ balsa sheet and reproduce the broad, gentle compound curvature that’s already defined by the formers and stringers. By measuring the model I determined that I would need to cut this piece from a 4″ wide sheet of balsa to avoid dealing with a seam partway across the surface that would make smooth bending just about impossible. I cut the 1/32″ sheet skin to fit neatly against the back of the laminated cowl front formers and extend back past the instrument panel AND around the sides beyond the longerons. I’ll talk about the reason for that in a bit.

 

Titebond aliphatic wood glue is my choice here. I’m going to WET the 1/32″ balsa skin to allow me to form it around that compound curve. The water-based Titebond and the wet balsa are going to be very happy in each other’s company. I’m using a small brush to get an even application of Titebond to every bit of structure the balsa skin will touch.

 

I’m using high quality masking tape to hold the wet balsa sheet TIGHT against the structure of the fuselage. Be aware that masking tape is not going to stick to wet balsa…I used long enough pieces of tape to wrap all the way around and overlap on the other side. I started with the back edge to create a secure base that I can pull against to tighten up the rest of the sheet.

 

That wet-sheet forming job looks like this all stretched and pressed and taped securely into place. There is indeed a smooth convex curve bent into the balsa under all that tape…but…I had to fuss with it and re-tape several times to get it RIGHT before going off to let it dry. The result is going to be worth the trouble.

 

It looks like this with the tape off. You can see the subtle curvature at the upper edge. Notice that I let the skin extend down past the top longerons during forming. This extra material gave the tape something to work against to ensure that the surface curvature I wanted would continue (or “flow”) around and past the structural edge created by the longeron. I’ll cut that extra wood off soon.

 

It turns out that if I separate the lower portion of the cowl into front and rear sections I can avoid having to wet-form any more compound curves. Here I’m checking a paper pattern I made for an exact fit where the lower boot cowl skin will go.

 

I traced that pattern onto more 1/32″ balsa sheet, cut it out, and trimmed as necessary for a precise fit, and then used more Titebond and tape to attach it. Note that I used water to wet this skin panel as well…even with only simple curvatures sheet balsa conforms to structural curves WAY better than if you were to apply it dry. (It’s also WAY less likely to crack) There some added benefits to doing this that I’ll talk about another time.

 

This is the entire nose/cowl structure fully sheeted with 1/32″ balsa skin. I added the bottom front section just the same way as I did the rear.

 

Now it’s time for some shaping. I’m starting by making some rough cuts with my No. 11 blade.

 

The sanding block is the next step in that shaping job. The trick here is to cut/sand away exactly the right amount of balsa to replicate the shape defined by the plan and/or the scale drawings. Perhaps surprisingly, the most common error in this part of scale model airplane building is to take off too little material and leave a crude, lumpy surface.

 

The main body of the cowl is sanded to shape here so I can cut off the front (engine cowl) portion to get at the inside. The very front part of the cowl (the “nose bowl”) still needs some final shaping as well as hollowing.

 

More work for the razor saw. I’m cutting along carefully placed pencil marks that define the parting line at the rear of the engine cowl.

 

Cut off, it looks like this. Now I can work on the inside to create the firewall, motor mount, and other goodies. This is where the cowl parting line will be on the finished model.

 

I needed to design, make and install an entirely new firewall/motor mount former to finish off what has become the front of the primary fuselage structure,, and I had to make a judgment call here. I’m going to assume that plenty of you guys reading this will be interested in doing your own conversions, but you probably won’t all want to build the Dumas Piper J-4. I’m showing you how I figured out a pattern for the new firewall face based on the existing fuselage structure and what I wanted to do with it. Read on and more of the details will be made clear.

 

I cut out the pattern I just drew, used spray adhesive to attach it to some backing paper for support, and then contacted it to a piece of the 1/64″ plywood I’m using for the new part. The next few images will show you how it looked after I ZAP’d it into place against the existing structure.

 

I added a pair of inside fuselage side rails made from 1/4″ x 3/8″ balsa with a 1/16″ groove/slot cut 1/8″ deep into one face with my Dremel mini table saw. You can see the long 1/16″ plywood floor/tray that’s dimensioned to slip into those slots and slide into (and out of) the fuselage.

 

I’m going to have the rear portion of that tray installed semi-permanently in the fuselage to serve as a rudder-elevator servo mount. I cut off the tray as you can see here and mounted the servos with it outside the model where I could get at it all. Watch what happens next.

 

I inserted the servo mount tray into those 1/16″ slots and slid the entire assembly back into the fuselage to the point I’d previously marked on the rails to locate the servos exactly where I wanted them. I could have glued the tray in place there, but I want it to be removable. Watch what’s going to happen.

 

I added a pair of short pieces of 1/4″ triangle stock I cut from a piece of basswood stick. These are attached to the side rails but NOT to the servo tray. Here I’m drilling a 3/64″ pilot hole into one block and through the servo tray in line with one of the cabin side windows that’s going to be left open for access.

 

See how that all works? I’ll be able to reach through those open cabin side windows with this small Allen wrench to remove or replace the servo tray whenever I need to, but it won’t move otherwise and will provide a secure base for rudder and elevator control forces.Next time around I’ll show you how I designed and built the motor mount.

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Converting the 30in Rubber Powered Dumas Piper J-4 to Electric – Part 1
Converting the 30in Rubber Powered Dumas Piper J-4 to Electric – Part 2
Converting the 30in Rubber Powered Dumas Piper J-4 to Electric – Part 3
Converting the 30in Rubber Powered Dumas Piper J-4 to Electric – Part 4